Conventional antenna-less infrared detectors--for example bolometers, photovoltaic detectors (diodes), and photoconductive detectors, depend upon effective absorption of incident radiation. This requirement for effective absorption imposes a minimum thickness upon the design of element. In the case of photovoltaic and photoconductive detection, (ie photodetectors) intrinsic semiconductor materials are commonly employed.
The infrared optical absorption coefficients, .alpha., for such materials lie typically in the range 10.sup.3 to 10.sup.4 cm.sup.-1. The element thickness needs to be of order or greater than (1/.alpha.). For the range of cadmium mercury telluride ternary alloy materials (Cd.sub.x Hg.sub.1-x Te), this implies an absorption thickness (ie 1/.alpha.) of 10 .mu.m or thereabouts. Extrinsic materials have also been employed. However, the absorption mechanism is much less efficient.
In many instances of application, photodetector performance--in particular the detectivity--is limited by generation-recombination (g-r) noise. This limitation arises, for example, whenever the background flux falling on the detector is very low, or when the detectors cannot be cooled to very low temperatures. Generation-recombination noise in semiconductors is a volume-dependent effect, in fact proportional to square root of volume. Because the requisite detector thickness is high, the noise in consequence is high and the detectivity poor. Also excess carriers generated by photoconversion are distributed over a large volume, and in consequence both excess carrier concentration and responsivity are low. In photoconductive detectors, furthermore, the high thickness implies low resistance and, in consequence, high power dissipation in bias field.
Nonlinear diode, metal whisker, antenna-coupled detectors, both metal-insulator-metal and metal-semiconductor junction devices, have been reported. [See Hocker et al, Appl. Phys. Lett. Vol. 12, No. 12, pp. 401-402, Jun. 15 (1968); and, Tsang et al, Appl. Phys. Lett Vol. 30, No. 6, pp. 263-265, Mar. 15 (1977)]. In these detectors the antennae collect energy from the radiation field and apply it, in the form of a voltage at infrared frequency, to the junctions. This has the effect of producing a change in dc voltage or current. It has been interpreted in terms of rectification of the infrared frequency current, due to nonlinearity of the junction's current-voltage characteristic. [See Faris et al, Appl. Phys. Lett. Vol. 27, No. 11, pp. 629-631, Dec. 1 (1975)]. A more general theoretical study of antenna-coupled infrared detectors has been reported by Schwartz et al. [J. Appl. Phys. Vol. 48 No 5 pp 1870-3 (May 1977)]. This study has shown that noise equivalent power can be reduced considerably by careful design of bolometers. The bolometer detailed comprises a number of thermally sensitive elements interconnected by conductive links, which links are also configured to provide antenna-coupling. The study also details a metal whisker photovoltaic detector, a sharpened tungsten whisker in point contact with infrared photoresponsive semiconductor material. The whisker/semiconductor material combination is a point contact diode since a rectifying contact is provided. Signals are extracted via the whisker. Moreover, the whisker has antenna-like properties at frequencies appropriate to its linear dimensions. To produce an antenna-coupled infrared detector from such an arrangement, it is necessary that field following properties be obtained; ie the detector must follow and rectify signals at infrared frequency. This imposes severe constraints, such as the need for very small geometries to provide low capacitance. The result is high inductance and high resistance with low efficiency and consequent noise. The above-mentioned expert study concluded that it was doubtful that any useful reduction in noise could be obtained for photodetectors by antenna coupling.